Limbed vertebrates have functionally integrated postcranial axial and appendicular systems derived from two distinct populations of embryonic mesoderm. The axial skeletal elements arise from the paraxial somites, the appendicular skeleton and sternum arise from the somatic lateral plate mesoderm, and all of the muscles for both systems arise from the somites. Recent studies in amniotes demonstrate that the scapula has a mixed mesodermal origin.Here we determine the relative contribution of somitic and lateral plate mesoderm to the avian scapula from chick/quail chimeras. We generate 3D reconstructions of the grafted tissue in the host revealing a very different distribution of somitic cells in the scapula than previously reported. This novel 3D visualization of the cryptic border between somitic and lateral plate populations reveals the dynamics of musculoskeletal morphogenesis and demonstrates the importance of 3D visualization of chimera data. Reconstructions of chimeras make clear three significant contrasts with existing models of scapular development. First, the majority of the avian scapula is lateral plate derived and the somitic contribution to the scapular blade is significantly smaller than in previous models. Second the segmentation of the somitic component of the blade is partially lost; and third, there are striking differences in growth rate between different tissues derived from the same somites that contribute to the structures of the cervical thoracic transition, including the scapula. These data call for the reassessment of theories on the development, homology, and evolution of the vertebrate scapula.

The movie above is a orthoslice composite, like a flip book. It shows consecutive sections through a chick embryo with a graft of quail somites at the shoulder region. The transplant was done at about 30 hours of development, and the chimera allowed to develop for 9 days. The chick cells are pink, the quail cells appear black because of antibody labeling.

This movie is a 3D reconstruction of skeletal structures generated from serial sections of a quail-chick chimera with a shoulder level graft. The chick cells are white, the quail cells are green.